Proppant with enhanced interparticle bonding

ABSTRACT

Polymer coated proppants for hydraulic fracturing of oil and gas wells have an outer layer portion that comprises an organofunctional coupling agent, preferably an organofunctional silane coupling agent. The use of an organofunctional silane coupling agent in the outer layer portion of the proppant coating is preferably chosen to expose functionalities that will be reactive towards similar functionalities of adjacent and similarly coated proppants so that, when introduced downhole, these proppants form interparticle bonds at the temperatures and crack closure pressures found downhole in fractured strata. Such enhanced interparticle bonding helps keep the proppant in the fracture and maintains conductivity with reduced flowback. The invention also helps proppants designed for low temperature well to bond more firmly and allows proppants designed for high temperature wells to bond well even at lower downhole temperatures, thereby extending their useful range.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/897,288, filed May 17, 2013, now U.S. Pat. No. 10,100,247 which ishereby incorporated by reference in its entirety.

FIELD OF INVENTION

The invention relates to a composition and method for the production ofproppants having an outer coating that promotes interparticle bonding.

BACKGROUND OF THE INVENTION

Coated proppants are often used in hydraulic well fracturing to increaseproduction rate of the well. The commercial “standard” coatings aretypically a form of phenolic thermoset coating. Partially cured phenolicproppants are typically used in low temperature wells (i.e., thosehaving bottom hole temperature of less than about 150° F. (66° C.))which typically exhibit low crack closure stresses (e.g., 2000-6000psi). The theory behind their use is that the residual reactivity of thepartially cured phenolic coating in conjunction with a surfactantactivator (which acts like a plasticizer) and the existence of waterfound in most wells will permit the coating to soften and flow, therebyallowing the proppants to consolidate and form interparticle bondsduring the “shut-in” period. The formation temperature of the downholeconditions is supposed to complete the curing reactions in situ in thepropped formation. An external activator fluid is used to soften theouter surface of these partially cured coated proppants in an effort toencourage consolidation and interparticle bonding. The activator itselfraises, however, additional issues of compatibility with the fracturingand breaker fluids as well as the possibility of adverse effects on thecontinued conductivity of the proppant packed fractured strata.

For high temperature wells, such as those with a bottom hole temperatureabove about 200° F. (93° C.), both partially cured phenolic coatedproppants (with an activator) or precured phenolic coatings are oftenused. In the case of the precured coaled proppant, the fracture/crackclosure stresses are often above 6,000 psi are used as the mainmechanism for holding proppant within the cracked strata.

In practice, however, a variety of factors can adversely affect theperformance and usefulness of the partially cured, phenolic coatings.The most important of these is premature curing of the partially curedphenolic resin in the coating due to exposure to high temperaturesbefore introduction into the fractured strata. Even the elevated,above-ground, temperatures found on loading docks and in shippingcontainers can be enough to effect curing of the coating long before itis desirable. This is particularly an issue that comes into play whenusing partially-cured phenolic coatings in deep high temperature wells.In low temperature applications, the partially cured phenolic coatingssimply take too long to cure to create bond strength. Bond strength canbe developed in a reasonable time frame if the activator is used inconjunction with the partially-cured, coated proppant. However, usingthe activator requires the addition to be metered into the fracturingfluid in a controlled manner at the right time. This increases thecomplexity of the fracturing treatment. Even if the activator is addedat the proper concentration and time, there still remain the issues withfracturing fluid compatibility and the aforementioned reduced fractureconductivity.

Two published patent applications discuss the use of isocyanates forproppant coatings. Tanguay et al. 2011/0297383 presents examples of hightemperature proppant coatings made of a polycarbodiimide coating onsand. The coating is said to be made from the reaction of a monomericisocyanate and a polymeric isocyanate. The catalyst is aphosphorous-based catalyst exemplified in example 1 by3-methyl-1-phenyl-2-phospholene oxide.

Tanguay et al. 2012/0018162 relates to a polyamide imide proppantcoating for high temperature applications. The examples have adescription of the use of polymeric diphenylmethane diisocyanate,trimellitic anhydride, one of three different types of amines,triethylamine as a catalyst, an adhesion promoter and a wetting agent.The coating/reaction process described lasts about 10 minutes followedby a post cure heating of 1-3 hours.

Recently, it has been discovered that cured, commercially acceptable,coatings can be applied to proppants using the polyurethane or polyureareaction products of polyols and isocyanates. The details of theseprocesses are disclosed in co-pending U.S. patent application Ser. No.13/099,893 (entitled “Coated and Cured Proppants”); Ser. No. 13/188,530(entitled “Coated and Cured Proppants”); Ser. No. 13/626,055 (entitled“Coated and Cured Proppants”); Ser. No. 13/224,726 (entitled “DualFunction Proppants”); Ser. No. 13/355,969 (entitled “Manufacture ofPolymer Coated Proppants”); and Ser. No. 13/837,396 (entitled “ProppantWith Polyurea-Type Coating”), the disclosures of which are hereinincorporated by reference. Commercially available proppants that usesuch coatings are available under the designations PEARL and GARNET fromPreferred Sands, Inc. Such polyurethane and polyurea based proppantcoatings are economically and environmentally desirable for a number ofreasons. Importantly, each acts like a fully cured coating for purposesof handling, shipping and introduction into a fractured field yetexhibit the inherent ability to form interparticle bonds under downholetemperatures and pressures for enhanced conductivity and proppantflowback control.

Despite the benefits found by the interparticle bonding seen in recentproppant coatings, there exists a continuing need in the industry for aproppant coating that will controllably form interparticle bond strengthat a wide variety of the expected downhole temperature and pressureconditions yet will not be compromised in forming such interparticlebond strength by premature exposure to elevated or high temperatures.

SUMMARY OF THE INVENTION

The present invention provides a polymer-coated proppant having an outerlayer portion that comprises an organofunctional coupling agent,preferably an organofunctional silane coupling agent. The use of anorganofunctional coupling agent in the outer layer portion of theproppant coating is preferably chosen to be reactive towards the polymerof the coating so that, when introduced downhole, the outer layer of theproppants are mutually reactive and form enhanced interparticle bondingat the temperatures and crack closure pressures found downhole infractured strata. Such enhanced interparticle bonding helps keep theproppant in the fracture and maintains conductivity with reducedflowback.

The coupling agent-enhanced proppant coating of the present inventionallows low temperature coatings to be much more effective in lowtemperature wells by increasing the exhibited interparticle bondstrength reflected by conventional UCS tests. It also allows hightemperature coatings to become useful for low and medium temperaturewells by exhibiting interparticle bond strength that was not previouslyexhibited or only marginally exhibited.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a proppant coating that includes atleast an outer layer portion that comprises an organic polymer coatingand a coupling agent. An inner layer portion that is immediatelyadjacent the core solid may be the made from the same polymer or adifferent polymer or resin. The inner and outer layer portions may beformed in one substantially continuous coating process (e.g., forming asingle type of polymeric coating or with changing proportions oringredients to form two different layer portions or polymers) or insequential coating processes (e.g., coating a cured or curable phenolicresin-coated proppant with a polyurethane or polyurea-based polymer toadd enhanced interparticle bonding). Preferably, a coupling agent isadded during the later stages of a single, substantially continuouscoating operation that forms one type of polymeric proppant coatinghaving an inner portion that is immediately adjacent the core solid andan outer layer portion that provides silane functionalities that areavailable for bonding with other, similarly coated proppants to form amass of proppants that exhibiting a bond strength in a conventional UCStest. Even more preferably, the silane coupling agent is added at anearly stage of the coating operation to help bond the coating to theproppant core and again later in the coating process to form an outerlayer that will provide enhanced interparticle bonding. It is alsopossible, however, to use one type of coupling agent for enhancing thebond between the proppant core solid and any applied coating and asecond, different type of coupling agent that is compatible with thepolymer coating for enhancing bond strength between adjacent proppantslodged within cracks of a fractured field.

Silanes usually have four main functions:

1. Crosslinking: Once attached to a polymer backbone, silanes can linkpolymer molecules together via the formation of siloxane bonds, creatinga three-dimensional network. This “crosslinking” is activated by ambientmoisture and can take place at ambient temperature. Silanes can provideimproved thermal stability, creep resistance, hardness and chemicalresistance to coatings, adhesives and sealants.

2. Adhesion Promotion: Silanes can provide improved substrate adhesionin adhesives, sealants and coatings, especially under hot and humidconditions. Silanes are commonly used to improve adhesion to glass andmetals, but they can also be beneficial with difficult substrates likepolyamide, SMC, acrylics, PVC and others.

3. Coupling: Silanes can couple inorganic pigments and fillers toorganic resins. Coupling typically improves the moisture and chemicalresistance of the coating or adhesive.

4. Dispersion: Silanes can aid in the dispersion of inorganic pigmentsand fillers in coatings and sealants. This can lead to lower viscosityin the formulated product and can improve the hiding power of a coating.

In the present invention, a coupling agent is added to the outer portionof an outer coating layer of the overall proppant coating to provideexposed coupling agent functionalities that will bond with exposedcoupling agent functionalities on adjacent proppants to enhanceinterparticle bonding between these adjacent, coated proppants underdownhole temperatures and crack closure stresses. Preferably,organofunctional silanes are used that are compatible with, or reactivetowards, the polymer or polymers used as the outer coating layer of theproppant. The organofunctional silanes are incorporated in a skinformation within the outer layers of the coating, in a targeted fashion.The alkoxysilane's functionality (organic reactivity) is designed tomatch the polymer system and optimize the reaction (grafting) with it,while exposing the alkoxy groups on the outer layers, and therefore onthe solid-air interface, which will increase tackiness andcrosslinking/bonding at the particle-to-particle contact point inpresence of water found downhole in fractured strata.

Organofunctional silanes are bi-functional molecules in that theyusually have two types of reactivity built into their structures—organicand inorganic. The organofunctional silanes that are useful in thepresent invention can exhibit any number of possiblefunctionalities—discrete moieties to polymers—provided that the addedfunctionalities are compatible with the polymer and/or polymericcomponents of the proppant coating. For example, preferred organicfunctionalities are compatible with the polyol component if the proppantcoating is a polyurethane polymer.

One type of organofunctional silane that is useful in the presentinvention can be represented by Formula 1 below which shows the commonelements of a typical organofunctional silane.X—R—Si—(Z1)(Z2)(Z3)  Formula 1

-   -   Wherein:    -   X=a reactive organic group    -   R=a linking group    -   Z1, Z2 and Z3=hydrolysable groups (Let me know if you want to        add silicates. I do not think they are coupling agents. If you        believe otherwise, let's discuss.)

The organic end (X) is designed for reactivity with an organic resin.Reactive organic groups that are available include primary andsubstituted amino, epoxy, methacryl, vinyl, mercapto, urea andisocyanate. The organic group is selected either to react with orco-polymerize into a resin or to take part in the cure reaction of theresin system.

Between the organic group and the silicon atom is a linking group,commonly, a “trimethylene chain.” The silicon-carbon bond of the linkinggroup is stable to most environmental conditions. The inorganic end ofthe molecule reacts through hydrolyzable groups attached to silicon (Z1,Z2, Z3). The hydrolyzable groups are usually alkoxy groups such asmethoxy, ethoxy or isopropoxy. Each hydrolyzes at a different rate andreleases a different alcohol upon reaction with ambient moisture. Insome cases, only two hydrolyzable groups are present, although athree-group configuration is more convenient synthetically and usuallygives more moisture-resistant bonds. Most coupling agents have only onesilicon atom, but some silanes are available with multiple silicons.

A preferred silane that is usable as a coupling agent can be anorganosilicon, which can be derived from an organic silane having thechemical structure of Formula 2

Within the chemical structure of Formula 2, the X can be a functionalgroup selected from the group consisting of: hydrogen, an amino group, apolyamino alkyl group, a mercapto group, a thiocyanato group, an epoxygroup, a vinyl group, a halogen, an acryloxy group, and a methacryloxygroup. Within the chemical structure, the Y can be an integer equal toor greater than 0. Within the chemical structure, the Z₁, Z₂, and Z₃ caneach be independently selected from the group consisting of hydrogen,alkoxy, halogen, and hydroxyl.

Suitable organofunctional silanes for use in the present invention havethe general structure of Formula 1 noted above. Exemplaryorganofunctional silane coupling agents that are useful for the presentinvention include those with the structure: Si(R¹)(R²)(R³)(R⁴), in whichR¹, R², R³, and R⁴ may the same or different and are independentlyselected from the group consisting of hydrogen, alkyl, haloalkyl,alkylene, alkynyl, alkoxy, alkynoxy, aryl, aryloxy, substitutedaromatic, heteroaromatic, amino, aminoalkyl, arylamino, epoxide, thiol,and haloalkyl, ether, ester, urethane, amide, provided that at least oneof R¹, R², R³, and R⁴ comprises an organic moiety. Preferably, theoganofunctional silane coupling agent includes an organic functionalityselected from the group consisting of methyl, epoxide, melaminefunctionalized with an epoxide or copolymerized with an epoxy, amino,mercapto, chloropropyl, methacryl, methacryloxy, vinyl, benzylamino,ureido, tetrasulfido, and C1-C4 alkoxy groups. Even more preferably, theorganofunctional silane is selected from the group consisting ofmercaptosilanes possessing at least one hydroxyalkoxysilyl group and/ora cyclic dialkoxysilyl group, blocked mercaptosilane possessing at leastone hydroxyalkoxysilyl group and/or a cyclic dialkoxysilyl group;mercaptosilanes in which the silicon atoms of the mercaptosilane unitsare bonded to each other through a bridging dialkoxy group, each silaneunit optionally possessing at least one hydroxyalkoxysilyl group or acyclic dialkoxysilyl group; blocked mercaptosilane dimers in which thesilicon atoms of the blocked mercaptosilane units are bonded to eachother through a bridging dialkoxy group, each silane unit optionallypossessing at least one hydroxyalkoxysilyl group or a cyclicdialkoxysilyl group; silane dimers possessing a mercaptosilane unit thesilicon atom of which is bonded to the silicon atom of a blockedmercaptosilane unit through a bridging dialkoxy group, each silane unitoptionally possessing at least one hydroxyalkoxysilyl group or a cyclicdialkoxysilyl group; mercaptosilane oligomers in which the silicon atomsof adjacent mercaptosilane units are bonded to each other through abridging dialkoxy group, the terminal mercaptosilane units possessing atleast one hydroxyalkoxysilyl group or a cyclic dialkoxysilyl group;blocked mercaptosilane oligomers in which the silicon atoms of adjacentblocked mercaptosilane units are bonded to each other through a bridgingdialkoxy group, the terminal mercaptosilane units possessing at leastone hydroxyalkoxysilyl group or a cyclic dialkoxysilyl group; and silaneoligomers possessing at least one mercaptosilane unit and at least oneblocked mercaptosilane unit, the silicon atoms of adjacent silane unitsbeing bonded to each other through a bridging dialkoxy group, theterminal silane units possessing at least one hydroxyalkoxysilyl groupor a cyclic dialkoxysilyl group.

Specific examples of useful organofunctional silane coupling agents foruse in the outer coating layer of proppants according to the inventioninclude 3-glycidyloxypropyltrimethoxysilane,3-glycidyloxypropyltriethoxysilane,2-(3,4-epoxycyclohexy)ethyltrimethoxysilane, and2-(3,4-epoxycyclohexyl)ethyltriethoxysilane;3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane (CAS No.35141-30-1); 3-mercaptopropyl-trimethoxysilane (CAS No. 4420-74-0);n-propyltrimethoxysilane (CAS No. 1067-25-0);[3-(2-aminoethyl)aminopropyl]trimethoxysilane (CAS No. 1760-24-3);silane n-dodecyltrimethoxysilane (CAS No. 3069-21-4);bis(trimethoxysilylpropyl) amine (CAS No. 82985-35-1);1,2-bis(trimethoxysilyl)ethane (CAS No. 18406-41-2);vinyltri(2-methoxyethoxy) silane (CAS No. 1067-53-4);n-octyltriethoxysilane (CAS No. 2943-75-1); bis[3-(triethoxysilyl)propyl]tetrasulfide (CAS No. 40372-72-3); vinyltriethoxysilane (CAS No.78-08-0): 3-glycidoxypropyl-trimethoxysilane (CAS No. 2530-83-8);3-mercaptopropyl-triethoxysilane (CAS No. 14814-09-6);3-glycidoxypropyl-triethoxysilane (CAS No. 2602-34-8);2-(3,4-epoxycyclohexyl)ethyl]trimethoxysilane (CAS No. 3388-04-3);3-aminopropyltrimethoxysilane (CAS No. 13822-56-5);2-(3,4-epoxycyclohexyl)ethyl]triethoxysilane (CAS No. 10217-34-2);3-aminopropyltriethoxysilane (CAS No. 919-30-2);3-glycidoxypropyl-methyldimethoxysilane (CAS No. 65799-47-5);bis(triethoxysilylpropyl)amine (CAS No. 13497-18-2);3-(2-aminoethylamino)propyldimethoxymethylsilane (CAS No. 3069-29-2);N-(n-Butyl)-3-aminopropyltri-methoxysilane (CAS NO. 31024-56-3);n-propyltriethoxysilane (CAS No. 2550-02-9); vinyltrimethoxysilane (CASNo. 2768-02-7); 3-ureidopropyltriethoxy-silane (CAS No. 23779-32-0);3-methacryloxypropyl-trimethoxysilane (CAS No. 2530-85-0).

Amine-functional silane compounds are especially preferred as adhesionpromoters in the present invention. See U.S. Pat. Nos. 6,071,990 and8,349,911 the disclosures of which are hereby incorporated by reference.Such amine-functional silanes are preferably (aminoalkyl) alkoxysilanesrepresented by the general Formula 3:(R4)NH(R3)Si(R2)_(b)(O)(R1)_(3-b)  Formula 3:

wherein R1 and R2 are monovalent hydrocarbon groups having 1 to 4carbons independently selected from a group comprising methyl, ethyl,propyl or butyl, and

b has a value of 0 or 1;

R3 is a divalent hydrocarbon group represented by the formula (CH₂),wherein x is a positive integer of from 3 to 10; and

R4 is selected from the group comprising hydrogen, a monovalenthydrocarbon group having one to four carbons or a group represented bythe formula, (R5)NH(R3) wherein R3 is as defined above and R5 ishydrogen or a monovalent hydrocarbon group having 1 to 4 carbons.Examples of groups represented by R4 comprise methyl, ethyl, propyl,butyl, aminomethyl, aminoethyl, aminopropyl, aminobutyl,ethylaminopropyl and aminoethylaminoethyl.

Examples of commonly available amine functional silane compounds theabove general formulae include aminopropyltriethoxysilane (DYNASYLAN®AMEO from Evonik Industries), aminopropyltrimethoxysilane (DYNASYLAN®AMMO), aminoethylaminopropyltrimethoxysilane (DYNASYLAN® AEAPTMS), andaminoethylaminoethylaminopropyltrimethoxysilane (DYNASYLAN® TAS).

Another type of organofunctional silanes that are useful in the presentinvention are silane-terminated polymers, such as silane-terminatedpolyethers and polyurethanes. These polymers are formed by reaction offor instance a polyether polymer with isocyanate termination withaminosilanes or a polyether polymer with amino termination and/orhydroxyl termination with isocyanate-terminated silanes. Reactions ofthe reactive groups with other materials in the composition is alsopossible to create other cross-links. Silane-terminated polymers (STP)or silane-modified polymers (MS) can be all pre-polymers which at thechain ends—or laterally—carry silyl groups having at least onehydroysable bond but which in the polymer framework, do not display thesiloxane bond (SiR2O)n that is typical of silicones. Two preferredsilane-terminated polymers are illustrated by Formulas 4 (adimethoxy(methyl)silylmethylcarbamate-terminated polyether) and 5:

Wherein for Formula 4: Polyether refers to a polyether chain having1-200 carbon atoms. See also published U.S. Pat. Nos. 3,971,751 and6,207,766 as well as US patent application publication number US2007/0088137, the disclosures of which are hereby incorporated byreference.

Wherein for Formula 5: R is an amine group; each X in Formula 5 can eachbe independently selected from the group consisting of hydrogen, alkoxy,halogen, and hydroxyl; and n is an integer that is greater than zero.Such agents are commercially available from Wacker Chemie AG,Hanns-Seidel-Platz 4, 81737 München, Germany under the designationGeniosil® STP-E.

The dipodal silane-terminated polyether-based polymers of Formulas 4-5are compatible or miscible with polyether polyols that can be used asthe polyol component for making a polyurethane proppant coating. Suchsilane-terminated polyether-based polymers are easily blended withpolyether polyols as a last step top-coat to provide an adhesive coatinglayer for coated proppants according to the invention. The dipodal aminosilane of Formula 4 in the form of bis(trimethoxysilylpropyl)amine hasbeen used as a coupling agent in the proppants industry for “difficult”substrates. In the present invention, this silane could provide twosilane, adhesive-like, functionalities for every amine (—NC═O) graftingmoiety.

The length of the carbon chain in the alkoxy moieties (e.g., methoxy vs.ethoxy vs. propoxy vs. butoxy) determine the rate of hydrolysis of thesilane. So, the choice of the length of the alkoxy carbon chain can beused to provide control over the resulting moisture and waterresistance. Increasing resistance is seen as the alkyl chain increases.Longer carbon length chains will also delay the hydrolysis and,therefore, the bonding performance of the proppant in the fracture.

A wide variety of polymers can be used as the inner and/or outer coatinglayers for proppants of the present invention. Indeed, the coatinglayers can be thermoset or thermoplastic and may the same, different,analogues or homologues of the other and any intervening proppantcoating layers. Exemplary polymer coatings include polyurethanes,polyurea-type polymers, phenolic resins, phenol-formaldehyde resins, andpolycarbodiimides.

The preferred proppant coatings for the present invention and theirmanufacture are polyurethane and polyurea-type coatings. These coatingsare described in more detail in co-pending U.S. patent application Ser.No. 13/099,893 (entitled “Coated and Cured Proppants”); Ser. No.13/188,530 (entitled “Coated and Cured Proppants”); Ser. No. 13/626,055(entitled “Coated and Cured Proppants”); Ser. No. 13/224,726 (entitled“Dual Function Proppants”); Ser. No. 13/355,969 (entitled “Manufactureof Polymer Coated Proppants”); and Ser. No. 13/837,396 (entitled“Proppant With Polyurea-Type Coating”), the disclosures of which areherein incorporated by reference.

Particularly preferred proppant coatings as the inner and/or outerlayers are those using polyurea-based or polyurethane-based polymers.The polyurea-type coating is preferably formed on the proppant from adynamically reacting mixture that comprises an isocyanate, water and acuring agent (preferably an aqueous solution containing a curing agentor catalyst) that have been simultaneous contacted and mixed in thepresence of the proppant core. While not wishing to be bound by theoryof operation, the controlled rates of substantially simultaneous waterand isocyanate are believed to allow the water to form a reactive aminespecies from the isocyanate, which newly-formed amine then reacts withother, unconverted isocyanate to form the desired polyurea-type coatingdirectly on the outer surface of the proppant solid. Thus, thesimultaneous contact among the ingredients forms a reacting mixture thatpolymerizes to form a thin, hard, substantially foam-free coatingdirectly on the outer surface of the proppant core. If the sand has beenheated in advance of the contact, the reaction can proceed substantiallyto completion in less than about four minutes to form a hard,substantially fully-cured coating that does not require post-curing toform a tack-free or substantially tack-free outer surface.

Alternatively and less preferably, a polyurea-type coating can be formedon the proppant core by serially adding polyurea-type precursorcomponents to the mixer. Such a process would likely need, however,sufficient agitation and mixing to avoid boundary layer effects from thefirst added component that would cover the surface of the proppant coreto a certain depth which might inhibit a complete reaction of all of thefirst material down to the surface of the proppant core solid.Sufficient agitation would be used to force the second component intothe boundary layer of first component so that the first componentboundary layer reacts downwardly from its outer surface towards theouter surface of the proppant core to form linkages that are tightlyadhered to the proppant core surface.

Similar concerns would occur if the proppant core had been stored underexternal conditions and had become wet. It would be desirable to heatthe proppant core above about 100° C., possibly less with moving airthrough the solids, until the proppants are substantially dry beforethey are first contacted with a reactable or reacting mixture ofpolyurea-type precursors. Such a drying process is commonly used inprocessing even uncoated sand proppants, the present coating process ispreferably performed in the same or adjacent facility as the dryingoperation so that the sensible heat introduced to the sand for dryingcan also be used to facilitate the formation of cured coatings on atleast a portion of the processed proppant sands.

Tests on the coating to determine its glass transition temperature (Tg)as well as laboratory-scale tests for bond strength, such asconventional UCS testing, or conductivity can be used to evaluate thesuitability of any particular coating formulation that has been preparedby a particular coating method. In particular, the Tg can be used as aguide to foretell whether a thermoplastic coating (such as those of thepresent invention, the polyurethanes described by our copending patentapplications that were noted above and incorporated by reference, orthose of the previously noted Tanguay et al. patent applications) arepotentially useable in the downhole conditions of a fractured stratum.It is desirable that the Tg of the proppant coating be a temperaturethat is less than that prevailing downhole so that the thermoplasticcoating has the ability to soften under prevailing combination oftemperature and pressure. For the present invention and for use in hightemperature wells, the Tg of the proppant coating is preferably greaterthan about 75° C. but less than about 200° C. and even more preferablywithin the range from about 100-165° C. For lower temperature wells,those with downhole temperatures within the range of 20°-52° C., the Tgof the proppant coating is desirably within the range of about 20° C. to60° C.

A preferred testing method for proppant performance is described in ISO13503-5:2006(E) “Procedures for measuring the long term conductivityproppants”, the disclosure of which is herein incorporated by reference.The ISO 13503-5:2006 provides standard testing procedures for evaluatingproppants used in hydraulic fracturing and gravel packing operations.ISO 13503-5:2006 provides a consistent methodology for testing performedon hydraulic fracturing and/or gravel packing proppants. The “proppants”mentioned henceforth in this part of ISO 13503-5:2006 refer to sand,ceramic media, resin-coated proppants, gravel packing media, and othermaterials used for hydraulic fracturing and gravel-packing operations.ISO 13503-5:2006 is not applicable for use in obtaining absolute valuesof proppant pack conductivities under downhole reservoir conditions, butit does serve as a consistent method by which such downhole conditionscan be simulated and compared in a laboratory setting.

The Isocyanate Component

The isocyanate-functional component for the coatings of the presentinvention comprises an isocyanate-functional component with at least 2reactive isocyanate groups. Other isocyanate-containing compounds may beused, if desired. Examples of suitable isocyanate with at least 2isocyanate groups an aliphatic or an aromatic isocyanate with at least 2isocyanate groups (e.g. a diisocyanate, triisocyanate ortetraisocyanate), or an oligomer or a polymer thereof can preferably beused. These isocyanates with at least 2 isocyanate groups can also becarbocyclic or heterocyclic and/or contain one or more heterocyclicgroups.

The isocyanate-functional component with at least 2 isocyanate groups ispreferably a compound, polymer or oligomer of compounds of the formula(III) or a compound of the formula (IV):

In the formulas (III) and (IV), A is each, independently, an aryl,heteroaryl, cycloalkyl or heterocycloalkyl. Preferably, A is each,independently, an aryl or cycloalkyl. More preferably A is each,independently, an aryl which is preferably phenyl, naphthyl oranthracenyl, and most preferably phenyl. Still more preferably A is aphenyl.

The above mentioned heteroaryl is preferably a heteroaryl with 5 or 6ring atoms, of which 1, 2 or 3 ring atoms are each, independently, anoxygen, sulfur or nitrogen atom and the other ring atoms are carbonatoms. More preferably the heteroaryl is selected among pyridinyl,thienyl, furyl, pyrrolyl, imidazolyl, pyrazolyl, pyrazinyl, pyrimidinyl,pyridazinyl, oxazolyl, isoxazolyl or furazanyl.

The above mentioned cycloalkyl is preferably a C₃₋₁₀-cycloalkyl, morepreferably a C₅₋₇-cycloalkyl.

The above mentioned heterocycloalkyl is preferably a heterocycloalkylwith 3 to 10 ring atoms (more preferably with 5 to 7 ring atoms), ofwhich one or more (1, 2 or 3) ring atoms are each, independently, anoxygen, sulfur or nitrogen atom and the other ring atoms are carbonatoms. More preferably the heterocycloalkyl is selected from amongtetrahydrofuranyl, piperidinyl, piperazinyl, aziridinyl, acetidinyl,pyrrolidinyl, imidazolidinyl, morpholinyl, pyrazolidinyl,tetrahydrothienyl, octahydroquinolinyl, octahydroisoquinolinyl,oxazolidinyl or isoxazolidinyl. Still more preferably, theheterocycloalkyl is selected from among tetrahydrofuranyl, piperidinyl,piperazinyl, pyrrolidinyl, imidazolidinyl, morpholinyl, pyrazolidinyl,tetrahydrothienyl, oxazolidinyl or isoxazolidinyl.

In the formulas (III) and (IV), each R¹ is, independently, a covalentbond or C₁₋₄-alkylene (e.g. methylene, ethylene, propylene or butylene).Preferably each R² is hydrogen or a covalent bond.

In the formulas (III) and (IV), each R² is each, independently,hydrogen, a halogen (e.g. F, Cl, Br or I), a C₁₋₄-alkyl (e.g. methyl,ethyl, propyl or butyl) or C₁₋₄-alkyoxy (e.g. methoxy, ethoxy, propoxyor butoxy). Preferably, each R² is, independently, hydrogen or aC₁₋₄-alkyl. More preferably each R² is hydrogen or methyl.

In the formula (IV), R³ is a covalent bond, a C₁₋₄-alkylene (e.g.methylene, ethylene, propylene or butylene) or a group—(CH₂)_(R31)—(O)—(CH₂)_(R32)—, wherein R31 and R32 are each,independently, 0, 1, 2 or 3. Preferably, R³ is a —CH₂— group or an —O—group.

In the formula (III), p is equal to 2, 3 or 4, preferably 2 or 3, morepreferably 2.

In the formulas (III) and (IV), each q is, independently, an integerfrom 0 to 4, preferably 0, 1 or 2. When q is equal to 0, thecorresponding group A has no substituent R², but has hydrogen atomsinstead of R².

In the formula (IV), each r and s are, independently, 0, 1, 2, 3 or 4,wherein the sum of r and s is equal to 2, 3 or 4. Preferably, each r ands are, independently, 0, 1 or 2, wherein the sum of r and s is equal to2. More preferably, r is equal to 1 and s is equal to 1.

Examples of the isocyanate with at least 2 isocyanate groups are:toluol-2,4-diisocyanate; toluol-2,6-diisocyanate;1,5-naphthalindiisocyanate; cumol-2,4-diisocyanate;4-methoxy-1,3-phenyldiisocyanate; 4-chloro-1,3-phenyldiisocyanate;diphenylmethane-4,4-diisocyanate; diphenylmethane-2,4-diisocyanate;diphenylmethane-2,2-diisocyanate; 4-bromo-1,3-phenyldiisocyanate;4-ethoxy-1,3-phenyl-diisocyanate; 2,4′-diisocyanate diphenylether;5,6-dimethyl-1,3-phenyl-diisocyanate; methylenediphenyl diisocyanate(including 2,2′-MDI, 2,4′-MDI and 4,4″-MDI);4,4-diisocyanato-diphenylether; 4,6-dimethyl-1,3-phenyldiisocyanate;9,10-anthracene-diisocyanate; 2,4,6-toluol triisocyanate;2,4,4′-triisocyanatodiphenylether, 1,4-tetramethylene diisocyanate;1,6-hexamethylene diisocyanate; 1,10-decamethylene-diisocyanate;1,3-cyclohexylene diisocyanate;4,4′-methylene-bis-(cyclohexylisocyanate); xylol diisocyanate;1-isocyanato-3-methyl-isocyanate-3,5,5-trimethylcyclohexane (isophoronediisocyanate); 1-3-bis(isocyanato-1-methylethyl) benzol (m-TMXDI);1,4-bis(isocyanato-1-methylethyl) benzol (p-TMXDI); oligomers orpolymers of the above mentioned isocyanate compounds; or mixtures of twoor more of the above mentioned isocyanate compounds or oligomers orpolymers thereof. A variety of polymeric isocyanates can be used in thepresent invention. Suitable examples include polymers and oligomers ofdiphenylmethane diisocyanates (MDIs and pMDIs), toluene diisocyanates(TDIs), hexamethylene diisocyanates (HDIs), isophorone diisocyanates(IPDIs), and combinations thereof. The preferred polymeric isocyanatefor use in the present invention is polymers and oligomers based ondiphenylmethane diisocyanates.

Particularly preferred isocyanates with at least 2 isocyanate groups aretoluol diisocyanate, methylenediphenyl diisocyanate, diphenylmethanediisocyanate, an oligomer based on toluol diisocyanate, an oligomerbased on methylenediphenyl diisocyanate (poly-MDI) or an oligomer basedon diphenylmethane diisocyanate and polymers thereof.

The Polyol Component

A polyol component with polyhydroxy functionality is one of thecomponents used in making a polyurethane coating on proppant solids in aprocess according to the invention, and it may be applied as the firstcomponent or the second component. The polyol component has two or morefunctional, hydroxyl moieties (such as diols, triols and higher polyolfunctionality based on starter molecules like glycerine,trimethylolpropane, sorbitol, methyl glucoside and sucrose) excludinghydroxyl groups associated with carboxylic acids and may or may not havereactive amine functionality. Preferred polyhydroxyl polyols includepolyethers (such as polyoxypropylene diols and triols), polyesters,aliphatic polyols, aromatic polyols, mixtures of aliphatic and aromaticpolyols, synthetic polyols, polyhydroxyoligomers (see U.S. Pat. Nos.4,554,188 and 4,465,815, the disclosures of which are herebyincorporated by reference), natural oil polyols (such as cashew nut oiland castor oil) and natural oils that have been treated to introducepolyhydroxyl content in place of unsaturated bonds such as oxidizedsoybean oil, oxidized peanut oil, and oxidized canola oil such aspolyols produced from biomass.

A preferred polyurethane coating is made with a polyol mixture thatincludes 5-100 wt % of one or more polyether, polyester, aliphaticand/or polyhydroxyoligomers polyols and 0-95 wt % of an aromatic polyol.An especially preferred polyol contains 0-50 wt % cashew nut oil, 0-60wt % aromatic polyol and 20-100%% castor oil. It appears that an optimumratio of castor oil to isocyanate in the coating mixture results inlower LOI loss as well as higher interparticle bond strength thancoatings with lower proportions of castor oil in the coating.

In a still further embodiment, the polyol component is a phenol resinwith monomer units based on cardol and/or cardanol. Cardol and cardanolare produced from cashew nut oil which is obtained from the seeds of thecashew nut tree. Cashew nut oil consists of about 90% anacardic acid andabout 10% cardol. By heat treatment in an acid environment, a mixture ofcardol and cardanol is obtained by decarboxylation of the anacardicacid. Cardol and cardanol have the structures shown below:

As shown in the illustration above, the hydrocarbon residue(—C₁₅H_(31-n)) in cardol and/or in cardanol can have one (n=2), two(n=4) or three (n=6) double bonds. Cardol specifically refers tocompound CAS-No. 57486-25-6 and cardanol specifically to compoundCAS-No. 37330-39-5.

Cardol and cardanol can each be used alone or at any particular mixingratio in the phenol resin. Decarboxylated cashew nut oil can also beused.

Cardol and/or cardanol can be condensed into the above described phenolresins, for example, into the resole- or novolak-type phenol resins. Forthis purpose, cardol and/or cardanol can be condensed e.g. with phenolor with one or more of the above defined compounds of the formula (I),and also with aldehydes, preferably formaldehyde.

The amount of cardol and/or cardanol which is condensed in the phenolresin is not particularly restricted and preferably is from about 1 wt %to about 99 wt %, more preferably about 5 wt % to about 60 wt %, andstill more preferably about 10 wt % to about 30 wt %, relative to 100 wt% of the amount of phenolic starting products used in the phenol resin.

In another embodiment, the polyol component is a phenol resin obtainedby condensation of cardol and/or cardanol with aldehydes, preferablyformaldehyde.

A phenol resin which contains monomer units based on cardol and/orcardanol as described above, or which can be obtained by condensation ofcardol and/or cardanol with aldehydes, has a particularly low viscosityand can thus preferably be employed with a low addition or withoutaddition of reactive thinners. Moreover, this kind of long-chain,substituted phenol resin is comparatively hydrophobic, which results ina favorable shelf life of the coated proppants obtained by the methodaccording to the present invention. In addition, a phenol resin of thiskind is also advantageous because cardol and cardanol are renewable rawmaterials.

Apart from the phenol resin, the polyol component can still containother compounds containing hydroxyl groups. The other compoundscontaining hydroxyl groups can be selected from the compounds containinghydroxyl groups that are known to be useful for making polyurethanes,e.g., hydroxy-functional polyethers, hydroxy-functional polyesters,alcohols or glycols. One preferred compound containing hydroxyl groupsis, for instance, castor oil. Compounds containing hydroxyl groups suchas alcohols or glycols, in particular cardol and/or cardanol, can beused as reactive thinners.

Curing Agents and Catalysts

The coatings of the invention can be cured with at least one of avariety of curing agents, including reactive, non-reactive (e.g.,“catalysts”) and partially reactive agents that facilitate the formationof polyurea-type linkages. Generally, the preferred curing agents areselected from the amine-based curing agents and are added to thereacting mixture of polyurea-type precursors at a total amount withinthe range from about 0.0001% to about 30 total wt %. The amine-basedcuring agents may also be used as a mixture of a fast-acting firstcuring agent and a second, latent curing agent if additionalcrosslinking ability is desired to take advantage of downhole heat andpressure conditions. Either of these first and/or second amine-basedcuring agents may be reactive, nonreactive or partially reactive. If theamine curing agent is reactive, however, the amine is preferably chosento favor the formation of polyurea by reaction with the isocyanate.Alkanolamines can also promote the formation of polyurea while alsoproviding hydroxyl functionality that will facilitate the incorporationof the added coupling agents in the outer layer of the proppant coatingaccording to the present invention.

Suitable single amine-based curing agent or a mixture of amine-basedcuring agents for promoting the formation of polyurea can include, butare not limited to, 2,2′-dimorpholinodiethyl ether,bis-dimethylaminoethylether; ethylene diamine; hexamethylene diamine;1-methyl-2,6-cyclohexyl diamine; 2,2,4- and2,4,4-trimethyl-1,6-hexanediamine;4,4-bis-(sec-butylamino)-dicyclohexylmethane and derivatives thereof;1,4-bis-(sec-butylamino)-cyclohexane;1,2-bis-(sec-butylamino)-cyclohexane; 4,4′-dicyclohexylmethane diamine;1,4-cyclohexane-bis-(methylamine); 1,3-cyclohexane-bis-(methylamine),isomers, and mixtures thereof; diethylene glycol bis-(aminopropyl)ether;2-methylpentamethylene diamine; diaminocyclohexane, isomers, andmixtures thereof; diethylene triamine; triethylene tetramine;tetraethylene pentamine; propylene diamine; 1,3-diaminopropane;dimethylamino propylamine; diethylamino propylamine;imido-bis-(propylamine); monoethanolamine, diethanolamine;triethanolamine; monoisopropanolamine, diisopropanolamine;isophoronediamine; 4,4′-methylenebis-(2-chloroaniline);3,5-dimethylthio-2,4-toluenediamine;3,5-dimethylthio-2,6-toluenediamine; 3,5-diethylthio-2,4-toluenediamine;3,5-diethylthio-2,6-toluenediamine; 4,4′-bis-(sec-butylamino)-benzene;and derivatives thereof; 1,4-bis-(sec-butylamino)-benzene;1,2-bis(sec-butylamino)-benzene; N,N′-dialkylamino-diphenylmethane;trimethyleneglycol-ci-p-aminobenzoate;polytetramethyleneoxide-di-p-aminobenzoate;4,4′-methylenebis-(3-chloro-2,6-diethyleneaniline);4,4′-methylenebis-(2,6-diethylaniline); meta-phenylenediamine;paraphenylenediamine; N,N′-diisopropyl-isophoronediamine;polyoxypropylene diamine; propylene oxide-based triamine;3,3′-dimethyl-4,4′-ciaminocyclohexylmethane; and mixtures thereof. Inone embodiment, the amine-terminated curing agent is4,4-bis-(sec-butylamino)-dicyclohexylmethane. Preferred amine-basedcuring agents and catalysts that aid the —NCO— and water reaction toform the polyurea-type links for use with the present invention includetriethylenediamine; bis(2-dimethylaminoethyl)ether;tetramethylethylenediamine; pentamethyldiethylenetriamine;1,3,5-tris(3-(dimethylamino)propyl)-hexahydro-s-triazine and othertertiary amine products of alkyleneamines.

Additionally, other catalysts that promote the reaction of isocyanateswith hydroxyls and amines that are known by the industry can be used inthe present invention, e.g., transition metal catalysts of Groups III orIV used for polyurea-type foams. Particularly preferred metal catalystsinclude dubutyltin dilaurate and added to water for application duringthe coating process.

Also preferred are catalysts that promote isocyanate trimerization overother reaction mechanisms. See, e.g., U.S. Pat. No. 5,264,572 (cesiumfluoride or tetraalkylammonuim fluoride), U.S. Pat. No. 3,817,939(organic carbonate salt), and U.S. Pat. No. 6,127,308 (lithium salts,lithium hydroxide, allophane catalysts such as tin-2-ethylhexanoate ortin octoate, and organic compounds containing at least one hydroxylgroup), the disclosures of which are herein incorporated by reference.Phosphorous-based catalysts have been used to promote the formation ofpolycarbodiimides (see the examples in Tanguay et al. US 2011/0297383)and are not preferred for use in the present invention.

The amine-based curing agent may have a molecular weight of about 64 orgreater. In one embodiment, the molecular weight of the amine-curingagent is about 2000 or less and is a primary or secondary amine.Tertiary amines will not generally be used as a reactant for formingpolyurea-type coatings.

Of the list above, the saturated amine-based curing agents suitable foruse to make polyurea-type coatings according to the present inventioninclude, but are not limited to, ethylene diamine; hexamethylenediamine; 1-methyl-2,6-cyclohexyl diamine; 2,2,4- and2,4,4-trimethyl-1,6-hexanediamine;4,4′-bis-(sec-butylamino)-dicyclohexylmethane;1,4-bis-(sec-butylamino)-cyclohexane;1,2-bis-(sec-butylamino-cyclohexane; derivatives of4,4′-bis-(sec-butylamino)-dicyclohexylmethane; 4,4′-dicyclohexylmethanediamine; 1,4-cyclohexane-bis-(methylamine);1,3-cyclohexane-bis-(methylamine); diethylene glycol bis-(aminopropyl)ether; 2-methylpentamethylene-diamine; diaminocyclohexane; diethylenetriamine; triethylene tetramine; tetraethylene pentamine; propylenediamine; dipropylene triamine; 1,3-diaminopropane; dimethylaminopropylamine; diethylamino propylamine; imido-bis-(propylamine);monoethanolamine, diethanolamine; monoisopropanolamine,diisopropanolamine; isophoronediamine; N,N′-diisopropylisophoronediamine and mixtures thereof.

In one embodiment, the curative used with the prepolymer include3,5-dimethylthio-2,4-toluenediamine,3,5-dimethyl-thio-2,6-toluenediamine,4,4′-bis-(sec-butylamino)-diphenylmethane, N,N′-diisopropyl-isophoronediamine; polyoxypropylene diamine; propylene oxide-based triamine;3,3′-dimethyl-4,4′-diaminocyclohexylmethane; and mixtures thereof.

Because unhindered primary diamines result in a rapid reaction betweenthe isocyanate groups and the amine groups, in certain instances, ahindered secondary diamine may be more suitable for use. Without beingbound to any particular theory, it is believed that an amine with a highlevel of stearic hindrance, e.g., a tertiary butyl group on the nitrogenatom, has a slower reaction rate than an amine with no hindrance or alow level of hindrance and further adds to the hydrolytic and thermalstability of the final product. For example,4,4′-bis-(sec-butylamino)-dicyclohexylmethane (CLEARLINK 1000® fromHuntsman Corporation in The Woodlands, Texas) may be suitable for use incombination with an isocyanate to form the polyurea-type coating. Inaddition, N,N′-diisopropyl-isophorone diamine, also available fromHuntsman Corporation, under the tradename JEFFLINK®, may be used as thesecondary diamine curing agent.

In addition, a trifunctional curing agent can be used to help improvecross-linking and, thus, to further improve the chemical and/or abrasionresistance of the coating. In one embodiment, a diethylene triamine ortriethylene tetramine are both highly reactive and are desirably addedto the coating process along with another, compatible component.

The curing agents of the present invention can be added to the coatingformulation simultaneously with any of the other components orpro-coated on the proppant. Preferably, the curing agent is co-appliedto the solid proppant core at substantially the same time thatisocyanate is added with any of the other components, e.g., adding thecuring agent with the polyol component.

Additives

The proppant coating compositions of the invention may also includevarious additives to the coating or the cured product that can changethe appearance, properties, handling characteristics or performance ofthe coating as a proppant or in fracturing or breaker fluids. Forexample, the coatings of the invention may also include pigments, tints,dyes, and fillers in an amount to provide visible coloration in thecoatings. Other materials include, but are not limited to, impactstrength enhancers, reinforcing agents, reaction rate enhancers orcatalysts, crosslinking agents, optical brighteners, propylenecarbonates, coloring agents, fluorescent agents, whitening agents, UVabsorbers, hindered amine light stabilizers, defoaming agents,processing aids, mica, talc, nano-fillers, silane coupling agents,anti-slip agents, water affinity or repulsion components,water-activated agents, viscosifiers, flowaids, anticaking agents,wetting agents, toughening agents such as one or more block copolymers,and components that act to remove at least some portion of the heavymetals and/or undesirable solutes found in subterranean groundwater.See, copending U.S. patent application Ser. No. 13/224,726 filed on 1Sep. 2011 entitled “Dual Function Proppants”, the disclosure of which isherein incorporated by reference.

The additives are preferably present in an amount of about 15 weightpercent or less. In one embodiment, the additive is present in anon-zero amount of about 5 percent or less by weight of the coatingcomposition.

The coated proppants can additionally be treated with surface-activeagents, anticaking agents, or auxiliaries, such as talcum powder orstearate or other processing aids such as fine amorphous silica toimprove pourability, wettability (even to the extent that a waterwetting surfactant can be eliminated), dispensability, reduced staticcharge, dusting tendencies and storage properties of the coated product.A preferred group of additives are anticaking agents that help thehandling characteristics of the coated and cured proppant to avoidagglomeration and to improve flow. Such anticaking agents includeamorphous silica (e.g., silica flour, fumed silica and silicadispersions) and silica alternatives (such as those used in sandblastingas an alternative to silica or organofunctional silane like theDYNASYLAN fluids from Evonik Degussa Corporation in Chester, Pa.). Theseagents are available as powders or dispersions. They are applied to theouter surfaces of the coated proppant solid to prevent the formation ofagglomerates during packing and shipping. Amorphous silica is preferablyapplied in an amount generally within the range from about 0.001 wt %about 1 wt % based on the dry proppant weight.

Adhesion promoters can be used to increase the bond strength between theouter surface of the proppant core solid and the applied polymericcoating. Silanes are a particularly preferred type of adhesion promoteragent that improves the affinity of the coating resin for the surface ofthe proppant core solid and is particularly useful when sand is theproppant core. Organofunctional titanates and zirconates may also beused when coating a ceramic core solid. The adhesion promoter can bemixed in as an additive during the initial stages of the coating processor applied as a separate pretreatment step for the core solid. Adhesionpromoters can also be converted in situ on from reactive constituents ofthe polyol component or of the isocyanate component used in the proppantcoating. Functional silanes such as amino-silanes, epoxy-, aryl- orvinyl silanes are commercially available. The amino-silanes arepreferred.

An optional additional additive is a contaminant removal component thatwill remove, sequester, chelate or otherwise clean at least onecontaminant, especially dissolved or otherwise ionic forms of heavymetals and naturally occurring radioactive materials (NORMS), fromsubterranean water or hydrocarbon deposits within a fractured stratumwhile also propping open cracks in said fractured stratum. Preferably,the contaminant removal component is associated with the proppant solidas a chemically distinct solid that is introduced together with theproppant solid as: (a) an insoluble solid secured to the outer or innersurface of the proppant solid with a coating formulation that binds thesolids together, (b) as a solid lodged within pores of the proppantsolid or (c) as a chemical compound or moiety that is mixed into orintegrated with a coating or the structure of the proppant solid. Seecopending U.S. patent application Ser. No. 13/224,726 filed on 2 Sep.2011 entitled “Dual Function Proppants” the disclosure of which isherein incorporated by reference. Additional added functionality canalso be in the form of fracture fluid breakers, de-emulsifiers, andbactericides.

The added functionality of an auxiliary particle to the proppant mayalso be in the form of an ion exchange resin that is pretreated or whichitself constitutes a dissolvable solid for the slow release of corrosionor scale inhibitors. Such slow release materials could prove beneficialand advantageous to the overall operation and maintenance of the well.

Other useful additives to the proppant coating or applied to the outersurface of the cured coating include scale inhibitors, paraffininhibitors, biocides, gel breakers, hydrogen sulfide scavengers orscavengers of other undesirable components found in fractured strata.

Proppant Core Solids

The proppants can be virtually any small solid with an adequate crushstrength and lack of chemical reactivity. Suitable examples includesand, ceramic particles (such as aluminum oxide, silicon dioxide,titanium dioxide, zinc oxide, zirconium dioxide, cerium dioxide,manganese dioxide, iron oxide, calcium oxide, magnesium oxide, orbauxite), or also other granular materials.

Proppant sands are a preferred type of proppant for the presentinvention. Sand is mainly used in the hydraulic fracturing process ofnatural gas and oil wells to increase their productivity of valuablenatural resources. Proppant sand is monocrystalline with a high silicacontent of at least 80 wt %, and more typically has a silica content ofgreater than about 97 wt % silica.

The American Petroleum Institute specifications place the followinglimitations on sieve distribution for proppants suitable for use inhydraulic fracturing:

-   -   At least 90% of material must fall between the two mesh sizes,    -   No more than 10% of the material may be coarser than the largest        mesh size,    -   No more than 0.1% of the material may be coarser than the next        largest mesh size, e.g. for 20/40, up to 10% of the proppant may        be between 16 and 20 mesh, but no more than 0.1% can exceed 16        mesh, and    -   No more than 1% of material is permitted to fall onto the pan.

Proppants are divided into low-density, medium density, high-densitywhen determined in bulk. Proppant crush strengths are divided into 52MPa, 69 MPa, 86 MPa and 103 MPa series. The size specifications ofproppant sand are generally 12-18 mesh, 12-20 mesh, 16-20 mesh, 16-30mesh, 20-40 mesh, between 30-50 mesh, 40-60 mesh, 40-70 mesh andsmaller. The proppants to be coated preferably have an average particlesize within the range from about 50 μm and about 3000 μm, and morepreferably within the range from about 100 μm to about 2000 μm.

Coating Method

The coating process of the present invention produces a polyurea-type orpolyurethane-type coating on the proppant core solids that is hard,durable and resists dissolution under the rigorous combination of highheat, agitation, abrasion and water found downhole in a fracturedsubterranean formation. Preferably, the cured coating exhibits asufficient resistance (as reflected by a 10 day autoclave test or 10 dayconductivity test) so that the coating resists loss by dissolution inhot water (“LOI loss”) of less than 25 wt %, more preferably less than15 wt %, and even more preferably a loss of less than 5 wt %. Thesubstantially cured coating of the invention thus resists dissolution inthe fractured stratum while also exhibiting sufficient consolidation andresistance to flow back without the use of an added bonding activatorwhile also exhibiting sufficiently high crush strength to prop open thefractures and maintain their conductivity for extended periods.

The preferred process for making polyurethane-type coatings is describedin copending U.S. patent application Ser. No. 13/355,969 (entitled“Manufacture of Polymer Coated Proppants”) which is hereby incorporatedby reference. Briefly summarized, the coating process of that inventionis the formation of a polyurethane-based coating on a solid proppantcore to form free-flowing, coated, proppant solids with the addition ofwater during the coating step as a processing aid for the formation ofthe coated proppants. Wafer is added at a rate or quantity sufficient tomaintain the discrete, free-flowing characteristics of the proppantsolids as the coating process proceeds yet avoid the formation ofsignificant amounts of foam. The water also helps to reduce or eliminatethe formation of viscous, resinous agglomerates or fouling masses ofproppant solids that may be coated with only one of the polyurethanereactants (e.g., the polyol or the isocyanate) or with both componentsbut at ratios that are not conducive to rapid formation of the desiredpolyurethane coating. Although the precise mechanism by which thiseffect occurs is not yet conclusively established so the presentinvention should not be bound by any particular theory of operation, theinventors believe that the water enables the reaction facilitates theformation of polyurea structures and reduces the number of unreacted—NCO bonds which elevates the thermal properties of the coating andhelps increase the crosslink density quickly. It may be that the wateracts as a reactive stabilizing fluid for these reactions because thefinal product is dry and free-flowing.

The temperature of the coating process is not particularly restrictedoutside of practical concerns for safety and component integrity. Thepreferred conditions for the coating/curing step of the presentinvention are generally at conditions within the range of about 50° toabout 175° C., more preferably at a temperature within the range fromabout 75° C. to about 150° C., and most preferably at a temperaturewithin the range from about 80° C. to about 135° C. As noted above, thistemperature is conveniently achieved by heating or using heated proppantsolids. The preferred temperature range avoids a number of emissionsissues, reduces the amount of energy consumed in the coating process andalso reduces the cooling time for the coated proppants for furtherhandling and packaging.

Mixing can be carried out on a continuous or discontinuous basis inseries or in several runs with a single mixer, but the specific mixerused to coat the proppants is not believed to be critical for thepresent invention. Suitable mixers include tumbling-type mixers, fluidbeds, a pug mill mixer or an agitation mixer can be used. For example, adrum mixer, a plate-type mixer, a tubular mixer, a trough mixer or aconical mixer can be used. The easiest way is mixing in a rotating drum.As continuous mixer, a worm gear can, for example, be used.

A preferred mixer type is a tumbling-type mixer that uses a rotatingdrum driven by an electrical motor. The load on the motor can be used asa measure of the viscosity of the tumbling solids and the degree towhich they are forming agglomerates or resinous deposits inside themixer; the electrical load on the motor increases as the agglomerationand fouling increase. Adding water to the mixing solids or adding one ormore of the polyurea precursor components in an aqueous solution,emulsion or suspension can help to reduce this load increase and retainthe free-flowing nature of the mixing solids, thereby enabling evenlarger productivity from the mixer.

As noted above in describing the formation of polyurea-type coatings,water is preferably added to the isocyanate at a rate sufficient to forma reactive amine species which then reacts almost immediately withadjacent isocyanate to form polyurea. Preferably, water and anisocyanate-containing component are used in an amount within the rangefrom about 5-30% water, 95-70% ISO consistent with the demands of thecatalyst to promote the hydrolysis of the ISO and temperature of thesubstrate during the timed additions onto the proppant substrate. Thewater and isocyanate are added at a rate sufficient to maintain aproportion of 5-30 to 95-70 so as to promote the in-situ formation of areactive amine component from the isocyanate which then reacts withunconverted isocyanate to make the polyurea-type coating of the presentinvention. These ratios also control the ultimate nature of the polyureaproduced, whether driven to pre-cured, or controlled to retain a levelof curability.

Most of the components for the coating are preferably added along witheither the water, the polyol or the isocyanate to facilitate propermixing and metering of the components. A silane adhesion promoter ispreferably added to the heated sand to prepare the surface for adhesionto the applied polymeric coating. A colorant is added during the coatingprocess by an injection line into the coating mixer. The amount addedand the timing of when the component is added can affect the bondstrength of the resulting coating to the core solid (adhesion promoteradded early or as core solid pretreatment) and/or the degree to whichinterparticle bonds between adjacent, coated proppants is formed(adhesion promoter added late in coating process). A surfactant and/orflow aid can be added after the proppants have been coated to enhancewettability and enhanced flow properties with lower fines generation,respectively.

The method for the production of coated proppants according to thepresent invention can be implemented without the use of solvents.Accordingly, the mixture obtained in step (a) in one embodiment of themethod is solvent-free, or essentially solvent-free. The mixture isessentially solvent-free, if it contains less than 20 wt %, preferablyless than 10 wt %, more preferably less than 5 wt %, and still morepreferably less than 3 wt %, and most preferably less than 1 wt %solvent, relative to the total mass of components of the mixture.

The coating is preferably performed at the same time as the curing ofthe coating on the proppant. In the present invention, the coatedproppant becomes free-flowing at a time of less than 5 minutes,preferably within the range of 1-41 minutes, more preferably within therange of 1-3 minutes, and most preferably within the range of 1-2minutes to form a coated, substantially cured, free-flowing, coatedproppant. This short cycle time combines with the relatively moderatecoating temperatures to form a coating/curing process that provideslower energy costs, smaller equipment, reduced emissions from theprocess and the associated scrubbing equipment, and overall increasedproduction for the coating facility.

The coating material or combinations of different coating materials maybe applied in more than one layer. For example, the coating process maybe repeated as necessary (e.g. 1-5 times, 2-4 times or 2-3 times) toobtain the desired coating thickness.

Alternatively, polyurea-type or polyurethane-type coatings containing anouter layer that incorporates an adhesion promoter according to theinvention can be applied as the outermost layer over an existingcoating, e.g., a precured or curable phenolic coating. Such a layeringprocess can take advantage of the underlying crush resistance and otherproperties of the phenolic coating while adding the bonding ability ofthe present polyurea-type or polyurethane-type coating. Such an outercoating would avoid the need for an added activator or surfactantcompounds that are typically required for the phenolic coatings used inlow temperature application, e.g., low temperatures well of 50° C. andgreater up to about 100° C. and thereby also avoid the potential forchemical incompatibility or interference with the formulated fracturingor breaker fluids used in hydraulic well fracturing. A typical sizerange for the final, coated proppant is desirably within the range ofabout 16 to about 100 mesh.

Polyurea-type and polyurethane-type coatings that contain anincorporated adhesion promoter can also be applied to a proppant thathas been previously coated with a polyurea or polyurethane, or formed insitu as the last step of a continuous coating process in what isbelieved to be an outermost “skin” layer of the proppant coating. Thisskin layer of coating may increase surface tackiness. The addition of ananticaking agent, such as silica flour or the like, can be used toretain free-flow properties in the resulting proppant.

Similarly, utilizing the high reactivity of this polyurea system, apolyurea can be formed as the basecoat, followed by a topcoat of aphenolic, or epoxy, polyurethane or other coating with an incorporatedadhesion promoter or coupling agent.

The amount of coating resin, that is, of the polyurea or polyurethanecomponents that are applied to a proppant, is preferably between about0.5 and about 10 wt %, more preferably between about 1% and about 5 wt%, resin relative to the mass of the proppant as 100 wt %. The amount ofadded adhesion promoter can be between about 00001 wt % to about 5 wt %relative to the mass of the proppant.

With the method according to the present invention proppants can becoated at temperatures between about 50° C. and about 175° C.,preferably within the range of about 75°-125° C. and preferably in asolvent-free manner. The coating process requires a comparatively littleequipment, and if necessary, can also be carried out near the sand orceramic substrate source, near the geographically location of theproducing field, or at/near the well itself.

If desired, and by no means is it required, the coated proppants can bebaked or heated for a period of time sufficient to further enhance theultimate performance of the coated particles and further react theavailable isocyanate, hydroxyl, amine and reactive adhesion promotergroups that might remain in the coated proppant. Such a post-coatingcure may occur even if additional contact time with a catalyst is usedafter a first coating layer or between layers. Typically, thepost-coating cure step is performed like a baking step at a temperaturewithin the range from about 100°-200° C. for a time of about 1 minute to4 hours, preferably the temperature is about 125-200° C. for about 1-30minutes.

Even more preferably, the coated proppant is cured for a time and underconditions sufficient to produce a coated proppant that exhibits a lossof coating of less than 25 wt %, preferably less than 15 wt %, and evenmore preferably less than 5 wt % when tested according to simulateddownhole conditions under ISO 13503-5:2006(E). Even more preferably, thecoated proppant of the present invention exhibits the low dust andhandling characteristics of a conventional pre-cured proppant (see APIRP 60) hut also exhibits a crush test result at 10,000 psi of less than10%, more preferably less than 5%, and especially less than 2%. Thecoated proppants of the invention preferably also have an unconfinedcompressive strength of greater than 20 psi and more preferably morethan 500 psi with a fracture conductivity at a given closure stress thatis substantially equal to, or greater than, the conductivity of aphenolic coating used in the same product application range.

Using the Coated Proppants

The invention also includes the use of the coated proppants inconjunction with a fracturing liquid to increase the production ofpetroleum or natural gas. Techniques for fracturing an unconsolidatedformation that include injection of consolidating fluids are also wellknown in the art. See U.S. Pat. No. 6,732,800 the disclosure of which isherein incorporated by reference. Generally speaking, a fluid isinjected through the wellbore into the formation at a pressure less thanthe fracturing pressure of the formation. The volume of consolidatingfluid to be injected into the formation is a function of the formationpore volume to be treated and the ability of the consolidating fluid topenetrate the formation and can be readily determined by one of ordinaryskill in the art. As a guideline, the formation volume to be treatedrelates to the height of the desired treated zone and the desired depthof penetration, and the depth of penetration is preferably at leastabout 30 cm radially into the formation. Please note that since theconsolidation fluid is injected through the perforations, the treatedzone actually stems from the aligned perforations.

Techniques for hydraulically fracturing a subterranean formation will beknown to persons of ordinary skill in the art, and will involve pumpingthe fracturing fluid into the borehole and out into the surroundingformation. The fluid pressure is above the minimum in situ rock stress,thus creating or extending fractures in the formation. In order tomaintain the fractures formed in the formation after the release of thefluid pressure, the fracturing fluid carries a proppant whose purpose isto prevent the fracturing from closing after pumping has been completed.

The fracturing liquid is not particularly restricted and can be selectedfrom among the fracturing liquids, foams and gases known in the specificfield. Suitable fracturing liquids are described, for example, in W CLyons, G J Plisga, “Standard Handbook of Petroleum And Natural GasEngineering,” Gulf Professional publishing (2005). The fracturing liquidcan be, for example, water gelled with polymers, an oil-in-wateremulsion gelled with polymers, a water-in-oil emulsion gelled withpolymers or liquefied petroleum gas. In one preferred embodiment, thefracturing liquid comprises the following constituents in the indicatedproportions: 1000 I water, 20 kg potassium chloride, 0.120 kg sodiumacetate, 3.6 kg guar gum (water-soluble polymer), sodium hydroxide (asneeded) to adjust a pH-value from 9 to 11, 0.120 kg sodium thiosulfate,0.180 kg ammonium persulfate and optionally a crosslinker such as sodiumborate or a combination of sodium borate and boric acid to enhanceviscosity.

In addition, the invention relates to a method for the production ofpetroleum or natural gas which comprises the injection of the coatedproppant into the fractured stratum with the fracturing liquid, i.e.,the injection of a fracturing liquid which contains the coated proppant,into a petroleum- or natural gas-bearing rock layer, and/or itsintroduction into a fracture in the rock layer bearing petroleum ornatural gas. The method is not particularly restricted and can beimplemented in the manner known in the specific field. The concentrationof proppant in the fracturing fluid can be any concentration known inthe art, and will typically be in the range of about 0.5 to about 20pounds of proppant added per gallon of clean fluid.

The fracturing fluid can contain an added proppant-retention agent, e.g.a fibrous material, a curable resin coated on the proppant, platelets,deformable particles, or a sticky proppant coating to trap proppantparticles in the fracture and prevent their production through thewellbore. Fibers, in concentration that preferably ranges from about0.1% to about 5.0% by weight of proppant, for example selected fromnatural organic fibers, synthetic organic fibers, glass fibers, carbonfibers, ceramic fibers, inorganic fibers, metal fibers and mixturesthereof, in combination with curable resin-coated proppants areparticularly preferred. The proppant-retention agent is intended to keepproppant solids in the fracture, and the proppant and proppant-retentionagent keep formation particles from being produced back out from thewell in a process known as “flowback.”

The enhanced proppants the present invention are particularly wellsuited for enhancing the interparticle bonding characteristics ofproppants designed for low temperature well, e.g., those with downholetemperatures in the range of about 75-125° F. and crack closure stresseswithin the range of about 3000-6000 psi. With the present invention,interparticle bonding can be enhanced from negligible bond strength togood bonding. Depending on the polymer used in the coating, the presentinvention can introduce improved bending of 100-1000% or more(alternatively, this might be measured as a bond strength increase of5-100 psi or more) relative to the same polymer without the addedcoupling agent in the outer layer portion of the proppant coating.

High temperature proppant coatings also benefit from the addition of acoupling agent in the outer layer portion of the coating. Such enhanced,high temperature coatings would normally be used in high temperaturewells having downhole conditions of 200-350° F. and 6000-12,000 psicrack closure stress levels. At 200° F., such proppants exhibit 0-5 psiof interparticle bonding strength according to conventional UCS testing.With the present invention, however, these same proppants will exhibitinterparticle bond strengths of 40-60 psi at 150° F. thereby extendingthe useful range across a wider range of downhole conditions.

EXAMPLE Example 1 Polyurea Coating Process

The following is an example of a sequence for making apolyurethane-based coating according to the invention having a silaneadhesion promoter between the proppant core and the inner layer of theproppant polymer coating and the same silane adhesion promoter addedinto the later duration of the addition sequence to incorporate theadhesion promoter into the outer layer of the proppant coating.

ADDITION CYCLE (Low temperature polyurethane coating)Time(start/stop)(m:s) Step 0:00 2000 g of preheated sand (200 F.) isadded to a lab mixer 0:00/0:05 1.2 g of aminopropyltriethoxysilanesilane is added with mixing over a 5 sec period 0:15/1:05 31.56 g ofpolyMDI is added over a 50 second period 0:20/0:55 28.44 g of a Polyoladded over a 35 second period 0:25/0:30 2 g of an oil based colorant isadded over a 5 second period 0:55/1:00 1.4 g ofaminopropyltriethoxysilane silane is added with mixing over a 5 secperiod 1:40/1:45 8.0 g of an aqueous solution of silica anticaking agent2:00 Coated sand is discharged at 190° F.

The advantage of the formulation in this example is the increased“bond-ability reflected in unconfined compressive strength (UCS) testsperformed on the resulting product due to the adhesive-like nature ofthe isocyanate-silane outer layer. The sticky nature of this particularcoating increases the risk of “caking” while in storage and handling sosilica anti-caking agent is added at the end of the cycle to protect theproduct from caking and agglomeration during storage.

Once those skilled in the art are taught the invention, many variationsand modifications are possible without departing from the inventiveconcepts disclosed herein. The invention, therefore, is not to berestricted except in the spirit of the appended claims.

What is claimed:
 1. A coated proppant comprising: a core and a coating,wherein the coating comprises: a first layer comprising a firstorganofunctional silane coupling agent adhered to the core; and a secondlayer comprising a polymer coupled to the first layer, wherein thesecond layer is admixed with a second organofunctional silane couplingagent and wherein the second organofunctional silane coupling agent isexposed on the outer surface of the coated proppant and can form bondswith other organofunctional silane coupling agents exposed on proppantsadjacent to the coated proppant.
 2. The coated proppant of claim 1,wherein the first and second organofunctional silane coupling agents canbe the same or different.
 3. A coated proppant according to claim 2wherein said first and second organofunctional silane coupling agent,independently, have a formula of Si(R₁)(R₂)(R₃)(R₄), in which R₁, R₂,R₃, and R₄ may the same or different and are, independently, selectedfrom the group consisting of hydrogen, alkyl, haloalkyl, alkylene,alkynyl, alkoxy, alkynoxy, aryl, aryloxy, substituted aromatic,heteroaromatic, amino, aminoalkyl, arylamino, epoxide, thiol, haloalkyl,ether, ester, urethane, and amide, provided that at least one of R₁, R₂,R₃, and R₄ comprises an organic moiety.
 4. A coated proppant accordingto claim 1, wherein said first and second organofunctional silanescomprise an organic functionality, independently, selected from thegroup consisting of methyl, amino, mercapto, chloropropyl, methacryl,methacryloxy, vinyl, benzylamino, ureido, tetrasulfido, and C₁-C₄ alkoxygroups.
 5. A coated proppant according to claim 1, wherein said firstand second organofunctional silanes are both an aminofunctional silane.6. A coated proppant according to claim 5, wherein said first and secondorganofunctional silanes are, independently, selected from the groupconsisting of aminopropyltriethoxysilane, aminopropyltrimethoxysilane,aminoethylaminopropyltrimethoxysilane, andaminoethylaminoethylaminopropyltrimethoxysilane.
 7. A coated proppantaccording to claim 1, wherein said first and second organofunctionalsilanes are, independently, selected from the group consisting of3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane (CAS No.35141-30-1); 3-mercaptopropyl-trimethoxysilane (CAS No. 4420-74-0);n-propyltrimethoxysilane (CAS No. 1067-25-0);[3-(2-aminoethyl)aminopropyl]trimethoxysilane (CAS No. 1760-24-3);silane n-dodecyltrimethoxysilane (CAS No. 3069-21-4);bis(trimethoxysilylpropyl) amine (CAS No. 82985-35-1);1,2-bis(trimethoxysilyl)ethane (CAS No. 18406-41-2);vinyltri(2-methoxyethoxy) silane (CAS No. 1067-53-4);n-octyltriethoxysilane (CAS No. 2943-75-1); bis[3-(triethoxysilyl)propyl]tetrasulfide (CAS No. 40372-72-3); vinyltriethoxysilane (CAS No.78-08-0): 3-glycidoxypropyl-trimethoxysilane (CAS No. 2530-83-8);3-mercaptopropyl-triethoxysilane (CAS No. 14814-09-6);3-glycidoxypropyl-triethoxysilane (CAS No. 2602-34-8);2-(3,4-epoxycyclohexyl)ethyl]trimethoxysilane (CAS No. 3388-04-3);3-aminopropyltrimethoxysilane (CAS No. 13822-56-5);2-(3,4-epoxycyclohexyl)ethyl]triethoxysilane (CAS No. 10217-34-2);3-aminopropyltriethoxysilane (CAS No. 919-30-2);3-glycidoxypropyl-methyldimethoxysilane (CAS No. 65799-47-5);bis(triethoxysilylpropyl)amine (CAS No. 13497-18-2);3-(2-aminoethylamino)propyldimethoxymethylsilane (CAS No. 3069-29-2);N-(n-Butyl)-3-aminopropyltri-methoxysilane (CAS No. 31024-56-3);n-propyltriethoxysilane (CAS No. 2550-02-9); vinyltrimethoxysilane (CASNo. 2768-02-7); 3-ureidopropyltriethoxy-silane (CAS No. 23779-32-0);3-methacryloxypropyl-trimethoxysilane (CAS No. 2530-85-0).
 8. A coatedproppant according to claim 1, wherein said first and secondorganofunctional silanes are, independently, selected from the groupconsisting of mercaptosilanes possessing at least one hydroxyalkoxysilylgroup and/or a cyclic dialkoxysilyl group; blocked mercaptosilanepossessing at least one hydroxyalkoxysilyl group and/or a cyclicdialkoxysilyl group; mercaptosilanes in which the silicon atoms of themercaptosilane units are bonded to each other through a bridgingdialkoxy group, each silane unit optionally possessing at least onehydroxyalkoxysilyl group or a cyclic dialkoxysilyl group; blockedmercaptosilane dimers in which the silicon atoms of the blockedmercaptosilane units are bonded to each other through a bridgingdialkoxy group, each silane unit optionally possessing at least onehydroxyalkoxysilyl group or a cyclic dialkoxysilyl group; silane dimerspossessing a mercaptosilane unit in which the silicon atom is bonded tothe silicon atom of a blocked mercaptosilane unit through a bridgingdialkoxy group, each silane unit optionally possessing at least onehydroxyalkoxysilyl group or a cyclic dialkoxysilyl group; mercaptosilaneoligomers in which the silicon atoms of adjacent mercaptosilane unitsare bonded to each other through a bridging dialkoxy group, the terminalmercaptosilane units possessing at least one hydroxyalkoxysilyl group ora cyclic dialkoxysilyl group; blocked mercaptosilane oligomers in whichthe silicon atoms of adjacent blocked mercaptosilane units are bonded toeach other through a bridging dialkoxy group, the terminalmercaptosilane units possessing at least one hydroxyalkoxysilyl group ora cyclic dialkoxysilyl group; and silane oligomers possessing at leastone mercaptosilane unit and at least one blocked mercaptosilane unit inwhich the silicon atoms of adjacent silane units being bonded to eachother through a bridging dialkoxy group, the terminal silane unitspossessing at least one hydroxyalkoxysilyl group or a cyclicdialkoxysilyl group.
 9. A coated proppant according to claim 1, whereinsaid first and second organofunctional silanes comprise a mixture ofsilanes.
 10. A coated proppant according to claim 1, wherein the polymeris a polyurethane-based polymer.
 11. A coated proppant according toclaim 1, wherein the polymer is a polyurea-based polymer.
 12. A coatedproppant according to claim 1, wherein said second organofunctionalsilane comprises a silane-terminated polymer.
 13. A coated proppantaccording to claim 12, wherein said second organofunctional silanecomprises a silane-terminated polyether or silane-terminatedpolyurethane.
 14. A coated proppant according to claim 12, wherein saidsecond organofunctional silane is a silane-terminated polyether.
 15. Acoated proppant according to claim 12, wherein said secondorganofunctional silane is a silane-terminated polyurethane.
 16. Amethod for propping open a fractured strata by a process that comprisesintroducing into said fractured strata a coated proppant according toclaim
 1. 17. A composition comprising a plurality of coated proppantsaccording to claim 1, wherein the coated proppants comprising the secondorganofunctional silane coupling agent exposed on the outer surface ofthe coated proppant binds to another organofunctional silane couplingagent exposed on an adjacent proppant comprising the secondorganofunctional silane coupling agent exposed on the outer surface.